In current cellular radio technologies, such as the LTE (Long Term Evolution) technology specified by 3GPP (3rd Generation Partnership Project), it is known to provide different mobility states a user equipment (UE) can be in. In the LTE technology, such mobility states are the idle state and the connected state, also referred to as RRC_IDLE state and RRC_CONNECTED states, respectively.
In the idle state, the UE keeps track of which cell it is located in and monitors a PDCCH (Physical Downlink Control Channel) in that cell for paging messages on specific paging occasions. This is also referred to as “camping” on the cell. The cellular network can contact the UE only on these specific paging occasions. On the other hand, the UE can access the cellular network using a random access procedure. This can be done each time when resources on a contention based physical random access channel (PRACH) are available, which typically occurs more frequently than the paging occasions. The UE performs measurements on surrounding cells and performs cell re-selection when needed in order to camp on a new cell. In the idle state the UE can move around within a certain area, consisting of the cells belonging to a Tracking Area (TA) in a TA list currently configured for the UE, without informing the cellular network. If the UE leaves the area corresponding to the configured TA list, it informs the cellular network through a Tracking Area Update (TAU). Further, the UE may also perform periodic TAUs to inform the cellular network that it is still reachable.
In the connected state the UE is connected to a certain cell, also referred to as serving cell, and monitors the PDCCH of this cell for downlink assignments addressed to the UE. The UE performs measurements on neighbor cells and when certain conditions are fulfilled the UE generate measurement reports which are sent to a base station of the cell, in the LTE technology referred to as eNB. Based on these measurements, the eNB may then decide to initiate a handover of the UE to a neighbor cell. In the connected state, the UE can access the network by sending a scheduling request (SR) on a PUCCH (Physical Uplink Control Channel) of the serving cell. If the UE has lost uplink synchronization, the UE may also access the cellular network through a random access procedure. In connected state the UE monitors the downlink control channel in every subframe, unless it has been configured with Discontinuous Reception (DRX). In connected state DRX, the UE monitors the downlink control channel only during regularly occurring active periods (which may be prolonged if data transmission/reception is ongoing), which are separated by usually longer inactive periods. Thus, during connected state DRX the network can reach the UE only during the active DRX periods. The UE, on the other hand, may access the network at any time using the methods described above. If no DRX is configured, the cellular network can contact the UE practically at any time via the PDCCH, and the UE can access the cellular network practically at any time via the PUCCH.
However, in certain scenarios the above-mentioned idle state and connected state may provide unsatisfactory results. Examples of such scenarios are semi-stationary UEs, e.g., UEs which are stationary but sometimes need to change cell due to shadowing or reflection of radio signals by close by objects, or UEs which are only moving in a limited area. While the idle state may be adequate as long as there is no transmission of data, it may cause excessive signaling overhead if transitions to the connected state are needed for transmission of data. This may for example happen in the case of MTC (Machine Type Communication) UEs, where typically only a small amount of data is transmitted on sparse occasions, or in the case of smartphone type UEs, which may generate burst-like traffic resulting in frequent changes between the idle state and the connected state.
In the case of completely stationary devices, the connected state in connection with DRX may allow for keeping the signaling overhead for sparse transmissions low. However, in the case of mobile UEs, the connected state may cause significant signaling overhead associated with the handovers between different cells. Further, such handovers may be problematic in the case of extended DRX sleep times, which may conflict with the frequent measurements and potential measurement reporting needed to support handovers.
Accordingly, there is a need for techniques which allow for efficiently controlling radio transmission in a cellular network with respect to efficient change of a UE between cells.